Draglines are commonly used for removing large volumes of material, such as dirt, loosened ore, etc., and are particularly well-suited for removing overburden in large strip mining operations where tens of millions of yards of material must be removed in an efficient manner. A typical dragline is shown in FIG. 1. Draglines work by dragging a large bucket along the surface to scoop up material, hence the name. Draglines provide several advantageous features over other earthmoving equipment, including a long reach for both digging and dumping, the ability to dig below their tracks (or base), and a high cycle speed.
Draglines are available in a variety of different sizes, with the largest draglines being among the most massive mobile equipment every produced. For example, the dragline shown in FIG. 1 is a Marion 8750 series dragline that has a 360 foot boom, and is equipped with a 135 cubic yard bucket. The largest dragline ever built has a bucket capacity of 220 cubic yards and weighs nearly 14,000 tons.
Referring to FIG. 1, the major components of a dragline include a powerplant 100, a boom 102, a hoist cable 104, a bucket 106, hoist chains 108, drag chains 110, dump cables 112, and drag cables 114. The machine powerplant 100 is mounted on a rotary base 115, allowing the boom to swing in the horizontal plane. Smaller draglines typically employ sets of tracks for moving the machine, while larger draglines use a "walking" mechanism. These larger machines are referred to as walking draglines. The hoist cable 104 can be retracted or extended by means of a hoist drum (not shown) that is located in the powerplant. Likewise, the drag cable 114 can be retracted and extended by means of a drag drum (not shown) located in the powerplant.
As shown in FIG. 2, the drag cable 114 is connected to pair of drag sockets 116. The drag sockets 116 are connected through drag devises 118 to the drag chains 110. The drag chains 110 are connected to the bucket 106 at hitch devises 120. The drag sockets 116 are also respectively connected to a pair of dump sockets 122 at dump devises 124. A second pair of dump sockets 126 is connected to the front of the bucket 106 at anchor links 128. The dump sockets 122 and 126 are commonly connected to a respective pair of dump cables 112 which ride on dump sheaves 130. A pair of upper hoist cables 132 are commonly connected to the bottom pickup link 134 at their top ends, and opposing sides of a spreader 136 at their bottom ends. A pair of lower hoist cables 138 are connected to the spreader 136 at their top ends, and are connected at their bottom ends to the bucket 106 at trunnions 139. The pickup link 134 is connected to a hoist equalizer 140, which in turn is connected to hoist sockets 142. The hoist sockets 142 are connected to the hoist cables 104. The hoist equalizer 140 is also connected to a pickup link 144, which is connected to a dump sheeve shackle assembly 146 that holds the dump sheaves 130.
The loads on the hoist and drag chain links are massive. It is common for the largest draglines to employ hoist and drag cables that are 5 inches in diameter. These cables are made out of very-high-strength steels, and support suspended loads exceeding 750,000 lbs. The loads placed on the hoist chains and drag chains are equally impressive. These loads dictate the use of specialized chain links made from ultra-high-strength alloyed steels. In addition, these chains and chain links must be designed to endure a tremendous amount of wear, as discussed below.
A typical dragline digging cycle is shown in FIGS. 3A-3F. As shown in FIG. 3A, the digging cycle begins by lowering the bucket into the mine pit with both the hoist cable and the drag cable nearly taut until the bucket contacts the pit surface. At this point the hoist cable is slightly slackened and the drag cable is pulled toward the tractor (FIGS. 3B-3E). This results in the bucket teeth digging in and cutting a slice of material that piles inside the bucket. The depth and angle of the cut may be controlled by varying the hoist cable length as the drag cable is pulled.
As a result of the digging operation, the drag chains are continuously dragged across and/or through the material being removed. This is particularly true for the chain links that are located closest to the bucket. In a typical dragline the size of the drag chain links are even more substantial than the size of either the hoist cable or the drag cable. This is due in part to the fact that the drag chain links must have sufficient surface areas to endure the constant wear that occurs during dragline digging operations. The ends of the chain links are also continuously worn as the chains are flexed during digging and dumpling operations. Dragline chains eventually become so worn that they fail and must be replaced, which is very costly in terms of both material and machine downtime. Large draglines are commonly operated 24 hours a day, seven days a week, and downtime cost for such machines may exceed $500 per minute.
The chain links are sized so that they will be able to support their loads after significant wear. The nominal size of the chain links (generally a cross-section width or thickness) is primarily a function of the strength of the chain link material, the load that must be carried, and empirical wear data. As a result of the wear considerations, conventional drag and hoist chains are sized to be much larger (and heavier) than would be necessary to carry their nominal loads.
The amount of material a dragline can remove (the payload) is primarily limited by the size of its bucket and the type of material the dragline is working in. The size of the bucket is limited by the maximum allowable suspended load rating of the machine, the suspended load including the weight of a loaded bucket and the weight of the various other components that are supported by the hoist cable (the hoist chains, drag chains, sockets, clevises, etc.--hereinafter the bucket support components). The suspended load rating is primarily a function of the strength of the boom, the torque capacity of the hoist drum and drag drum, and the overall horsepower of the machine.
The maximum suspended load rating for a machine is calculated by engineering analysis of the boom structure, using a safety factor that in part is determined by prior experience. It is generally desired to maximize the payload for a given machine, and this usually leads to using the machine at near its maximum suspended load rating. However, operating at near the maximum rating usually can only be performed on newer machines, because the strength of a boom is reduced over the lifetime of the dragline. This is due to the constant fatigue loading that is applied to the boom during machine operation. The fatigue loading of the boom can be reduced by reducing the suspended load. Unfortunately, a reduction in the suspended load usually means a reduction in payload.
It would therefore be advantageous to be able to (1) maximize the payload without reducing the suspended load and/or (2) reduce the suspended load without reducing the payload capacity. The first object can be accomplished by increasing the size of the bucket in conjunction with a decrease the weight of the bucket support components. The second object can be accomplished by simply reducing the weight of the bucket support components while maintaining the bucket size.
Both of these objects can be obtained by reducing the weight of the drag chains and/or hoist chains. The drag and hoist chains represent a significant portion of the total weight of the bucket support components. For instance, on a large machine individual drag chain links may weigh more than 300 lbs., and an entire drag chain may weigh upwards of 5 tons. In general, the weight of a loaded bucket can be increased by an amount equal to the reduction in drag chain and/or hoist chain weight. Alternatively, a reduction in the weight of the drag and/or hoist chains without a change in the size of the bucket will yield a commensurate decrease in the suspended load.
It is thus desired to reduce the weight of the drag and/or hoist chains. However, reduction of the weight of these chains has previously been limited because of the aforementioned wear considerations. It is therefore desired to produce reduced-weight drag and hoist chains that have similar performance characteristics when compared with heavier conventional chains.